2. Introduction
What is biomedical engineering?
Biomedical engineering is the interface
for research or development in the area
of medicine and engineering
3. • What is biomedical instrumentation?
The term biomedical engineering is used
for the field in general and the term
biomedical instrumentation for the
methods of measurement within the
field.
4. Development of Biomedical
Instrumentation
• Many instruments were developed as early as the nineteenth century—
for example, the electrocardiograph, first used by Einthoven at the end
of that century.
• In 1960s A large measure of help was provided by the U.S. government,
in particular by NASA (National Aeronautics and Space Administration).
• Also, in the 1960s, an awareness of the need for engineers and
technicians
to work with the medical profession developed. All the major
engineering
technical societies recognized this need by forming '^Engineering in
Medicine and Biology" subgroups, and new societies were organized.
5. Recent advances in biomedical
instrumentation
Trend #1: Improving Assistive Technologies
Trend #2: Improving Medical Imaging
Trend #3: Artificial Intelligence
Trend #4: Brain Research
Trend #5: Wearable Devices
6. Biometrics
The branch of science that includes the measurement of physiological
variables and parameters is known as biometrics.
7. In the design or specification of medical instrumentation systems, each of
the following factors should be considered:
• Range
• Sensitivity
• Linearity
• Hysterisis
• Frequency response
• Accuracy
• Signal to noise ratio
• Stability
• Isolation
• Symplicity
9. • The Subject
The subject is the human being on whom the measurements are made. Since it is
the subject who makes this system different from other instrumentation systems,
the major physiological systems that constitute the human body are treated in
much greater detail
• Stimulus
In many measurements, the response to some form of external stimulus is required.
The instrumentation used to generate and present this stimulus to the subject is a
vital part of the man-instrument system whenever responses are measured. The
stimulus may be visual (e.g., a flash of light), auditory (e.g., a tone), tactile (e.g., a
blow to the Achilles tendon), or direct electrical stimulation of some part of the
nervous system.
10. • The Transducer
In general, a transducer is defined as a device capable of converting one form of
energy or signal to another. In the man-instrument system, each transducer is used
to produce an electric signal that is an analog of the phenomenon being
measured. The transducer may measure temperature, pressure, flow, or any of the
other variables that can be found in the body, but its output is always an electric
signal. As indicated in Figure , two or more transducers may be used
imultaneously to obtain relative variations between phenomena.
• Signal-Conditioning Equipment
The part of the instrumentation system that amplifies, modifies, or in any other
way changes the electric output of the transducer is called signal conditioning (or
sometimes signal-processing) equipment. Signal-conditioning equipment is also
used to combine or relate the outputs of two or more transducers. Thus, for each
item of signal-conditioning equipment, both the input and the output are electric
signals, although the output signal is often greatly modified with respect to the
input. In essence, then, the purpose of the signal-conditioning equipment is to
process the signals from the transducers in order to satisfy the functions of the
system and to prepare signals suitable for operating the display or recording
equipment that follows.
11. • Display Equipment
To be meaningful, the electrical output of the signal-conditioning equipment
must be converted into a form that can be perceived by one of man‘s senses and that can
convey the information obtained by the measurement in a meaningful way. The input to
the display device is the modified electric signal from the signal-conditioning equipment.
Its output is some form of visual, audible, or possibly tactile information. In the man-
instrumentation system, the display equipment may include a gap
• Recording, Data-Processing, and Transmission Equipment
It is often necessary, or at least desirable, to record the measured information for possible
later use or to transmit it from one location to another, whether across the hall of the
hospital or halfway around the world. Equipment for these functions is often a vital part
of the man-instrument system. Also, where automatic storage or processing of data is
required, or where computer control is employed, an on-line analog or digital computer
may be part of the instrumentation system.hic pen recorder that produces a permanent
record of the data.
•Control Devices
Where it is necessary or desirable to have automatic control of the stimulus,
transducers, or any other part of the man-instrument system, a control system is
incorporated. This system usually consists of a feedback loop in which part of the
output from the signal-conditioning or display equipment is used to control the
operation of the system in some way.
12.
13. Transducers
•A device which converts a physical quantity into the proportional electrical signal
is called a transducer. The electrical signal produced may be a voltage, current or
frequency.
14. Quantity being
Measured
Input Device
(Sensor)
Output Device
(Actuator)
Light Level
Light Dependant Resistor (LDR)
Photodiode
Photo-transistor
Solar Cell
Lights & Lamps
LED’s & Displays
Fibre Optics
Temperature
Thermocouple
Thermistor
Thermostat
Resistive Temperature Detectors
Heater
Fan
Force/Pressure
Strain Gauge
Pressure Switch
Load Cells
Lifts & Jacks
Electromagnet
Vibration
Position
Potentiometer
Encoders
Reflective/Slotted Opto-switch
LVDT
Motor
Solenoid
Panel Meters
Speed
Tacho-generator
Reflective/Slotted Opto-coupler
Doppler Effect Sensors
AC and DC Motors
Stepper Motor
Brake
Sound
Carbon Microphone
Piezo-electric Crystal
Bell
Buzzer
Loudspeaker
15. A transducer uses many effects to produce such conversion. The process of
transforming signal from one form to other is called transduction. A transducer is
also called pick up.
The transduction element transforms the output of the sensor to an electrical
output, as shown in the Fig. A transducer will have basically two main
components. They are
1. Sensing Element : The physical quantity or its rate of change is sensed and
responded to by this part of the transistor.
2. Transduction Element : The output of the sensing element is passed on to the
transduction element. This element is responsible for converting the non-
electrical signal into its proportional electrical signal.
There may be cases when the transduction element performs the action of both
transduction and sensing. The best example of such a transducer is a
thermocouple. A thermocouple is used to generate a voltage corresponding to
the heat that is generated at the junction of two dissimilar metals.
16. The Classification of Transducers is done in many ways. Some of the criteria for the
classification are based on their area of application, Method of energy conversion,
Nature of output signal, According to Electrical principles involved, Electrical
parameter used, principle of operation, & Typical applications.
The transducers can be classified broadly as
i. On the basis of transduction form used
ii. As primary and secondary transducers
iii. As active and passive transducers
iv. As transducers and inverse transducers.
17. Types of Electrical Transducers
Mainly, the electrical transducers can be classified into the following two types.
•Active Transducers
•Passive Transducers
18. Active Transducers
The transducer, which can produce one of the electrical quantities such as voltage
and current is known as active transducer. It is also called self-generating
transducer, since it doesn’t require any external power supply.
The block diagram of active transducer is shown in below figure.
Following are the examples of active transducers.
•Piezo Electric Transducer
•Photo Electric Transducer
•Thermo Electric Transducer
19. It is a characteristic of active transducers that frequently, but not always, the same
transduction principle used to convert from a nonelectrical form of energy can also
be used in the reverse direction to convert electrical energy into nonelectrical forms.
For example, a magnetic loudspeaker can also be used in the opposite direction as a
microphone.
20. Piezo Electric Transducer:
An active transducer is said to be piezo electric transducer, when it produces
an electrical quantity which is equivalent to the pressure given to input. The
following three substances exhibit piezo electric effect.
•Quartz
•Rochelle salts
•Tourmaline
Quartz is used as piezo electric transducer, as it exhibits the moderate piezo
electric effect and having moderate mechanical strength among those three
piezo electric substances.
21. Quartz Transducer
The circuit diagram of Quartz transducer is shown in below figure. As shown in the
figure, quartz crystal is placed between base and force summing member. The
output voltage can be measured across the metal electrodes, which are placed on
two sides of quartz crystal.
The output voltage, Vo of above pressure transducer will be
Vo= Q / C
22.
23. Photo Electric Transducer
An active transducer is said to be photo electric transducer, when it produces an
electrical quantity which is equivalent to the illumination of light input. The circuit
diagram of photo electric transducer is shown in below figure.
The working of photo electric transducer is mentioned below.
•Step1 − The photo electric transducer releases electrons, when the light falls on cathode of
it.
•Step2 − The photo electric transducer produces a current, I in the circuit due to the
attraction of electrons towards anode. We can find the sensitivity of photo electric
transducer by using the following formula.
S = I/i
Where,
S - is the sensitivity of photo electric transducer
I - is the output current of photo electric transducer
i - is the illumination of the light input of photo electric transducer
24. The photoelectric transducer absorbs the radiation of light which falls on
their semiconductor material. The absorption of light energises the electrons
of the material, and hence the electrons start moving. The mobility of
electrons produces one of the three effects.
1. The resistance of the material changes.
2. The output current of the semiconductor changes.
3. The output voltage of the semiconductor changes.
Classification of Photoelectric Transducers
The photoelectric transducers are classified into following ways.
•Photoemissive Cell
The Photoemissive cell converts the photons into electric energy. It consists
the anode rode and the cathode plate. The anode and cathode are coated
with a Photoemissive material called caesium antimony.
25. When the radiation of light fall on cathode plates the electrons starts flowing from
anode to cathode. Both the anode and the cathode are sealed in a closed, opaque
evacuated tube. When the radiation of light fall on the sealed tube, the electrons starts
emitting from the cathode and moves towards the anode.
The anode is kept to the positive potential. Thus, the photoelectric current starts
flowing through the anode. The magnitude of the current is directly proportional to
the intensity of light passes through it.
26. •Photoconductive Cell
The photoconductive cell converts the light energy into an electric current. It uses
the semiconductor material like cadmium selenide, Ge, Se, as a photo sensing
element.
When the beam of light falls on the semiconductor material, their conductivity
increases and the material works like a closed switch. The current starts flowing into
the material and deflects the pointer of the meter.
27. •Photodiode
The photodiode is a semiconductor material which converts the light into the
current. The electrons of the semiconductor material start moving when the photodiode
absorbs the light energy. The response time of the photodiode is very less. It is designed
for working in reverse bias.
28. •Photo-voltaic cell
The photovoltaic cell is the type of active transducer. The current starts flowing into the
photovoltaic cell when the load is connected to it. The silicon and selenium are used as a
semiconductor material. When the semiconductor material absorbs heat, the free
electrons of the material starts moving. This phenomenon is known as the photovoltaic
effect.
The movements of electrons develop the current in the cell, and the current is known
as the photoelectric current.
29. •Phototransistor
The phototransistor is a device that converts the light energy into electric energy. It
produces both the current and voltage. The photovoltaic cell is a bipolar device which
is made of semiconductor material. The semiconductor material is enclosed in an
opaque container in which the light easily reaches to the photosensitive element. The
element absorbs light, and the current starts flowing from base to emitter of the
device. This current is converted into the voltages.
30. Thermo Electric Transducer
An active transducer is said to be thermo electric transducer, when it produces an
electrical quantity which is equivalent to temperature input. The following two
transducers are the examples of thermo electric transducers.
1. Thermistor Transducer
2. Thermocouple Transducer
1. Thermistor Transducer
The resistor, which depends on temperature is called thermal resistor. In short, it is
called Thermistor. The temperature coefficient of thermistor is negative. That means,
as temperature increases, the resistance of thermistor decreases. Mathematically, the
relation between resistance of thermistor and temperature can be represented as
31. Thermistor Transducer
The resistor, which depends on temperature is called thermal resistor. In short, it is
called Thermistor. The temperature coefficient of thermistor is negative. That means,
as temperature increases, the resistance of thermistor decreases. Mathematically,
the relation between resistance of thermistor and temperature can be represented
as
Where,
R1 is the resistance of thermistor at temperature T1⁰K
R2 is the resistance of thermistor at temperature T2⁰K
Β is the temperature constant
32.
33. Thermocouple Transducer
Thermocouple transducer produces an output voltage for a corresponding change of
temperature at the input. If two wires of different metals are joined together in order
to create two junctions, then that entire configuration is called Thermocouple. The
circuit diagram of basic thermocouple is shown below,
The above thermocouple has two metals, A & B and two junctions, 1 & 2. Consider a
constant reference temperature, T2 at junction 2. Let the temperature at junction, 1
be T1 . Thermocouple generates an emf (electro motive force), whenever the values
of T1 and T2 are different. That means, thermocouple generates an emf, whenever
there is a temperature difference between the two junctions, 1 & 2 and it is directly
proportional to the temperature difference between those two
junctions. Mathematically, it can be represented as
Where,
e is the emf generated by thermocouple
34. The above thermocouple circuit can be represented as shown in below figure for
practical applications.
The part of the circuit, which lies between hot & cold junctions including those two
junctions is an equivalent model of basic thermocouple. A PMMC galvanometer is
connected across the cold junction and it deflects according to the emf generated
across cold junction. Thermocouple transducer is the most commonly used
thermoelectric transducer.
35.
36. Magnetic Induction Type Transducers
When an electrical conductor moves in a magnetic field, it changes the magnetic flux
through the conductor. This produces a voltage, which is proportional to the rate of
change of flux. Induced EMF is given as
F = B*i*l
Where B is the magnetic induction, l is the length of the conductor, and V is the
velocity of he moving conductor. The negative sign indicates that the direction of
induced EMF and the direction of induced current are in the opposite direction.
The inverse magnetic effect is also true. When current passes through the electrical
conductor placed in the magnetic field, mechanical force ‘F’ acts on the conductor.
Applications Magnetic Induction Type Transducers
Electromagnetic flow meter
Heart sound Microphones
Indicating instruments
Pen motor in biomedical recorders
38. Passive Transducers
The transducer, which can’t produce the electrical quantities such as voltage and
current is known as passive transducer. But, it produces the variation in one of
passive elements like resistor (R), inductor (L) and capacitor (C). Passive
transducer requires external power supply.
The block diagram of passive transducer is shown in below figure.
Following are the examples of passive transducers.
1. Resistive Transducer
2. Inductive Transducer
3. Capacitive Transducer
39. Resistive Transducer
A passive transducer is said to be a resistive transducer, when it produces the
variation (change) in resistance value. the following formula for resistance, R of a
metal conductor.
Where,
‘ρ’ is the resistivity of conductor
‘l’ is the length of conductor
‘A’ is the cross sectional area of the conductor
The resistance value depends on the three parameters ρ,l & A. So, we can make
the resistive transducers based on the variation in one of the three parameters
42. Strain gage
Most transducers used for mechanical variables utilize a resistive element called the
strain gage. The principle of a strain gage can easily be understood with the help of
Figure) shows a cylindrical resistor element which has length, L, and cross-sectional
area, A. If it is made of a material having a resistivity of r ohm-cm, its resistance is
R = r*L/A ohms(n).
If an axial force is applied to the element to cause it to stretch, its length increases by an
amount, AL, as shown (exaggerated) in Figure (b). This stretching, on the other hand,
causes the cross-sectional area of the cylinder to decrease by an amount A A. Either an
increase in L or a decrease in A results in an increase in resistance. The ratio of the
resulting resistance change AR/R to the change in length aL/L is called the gage factor,
G. Thus;
The gage factor for metals is about 2, whereas the gage factor for silicon (a crystalline
semiconductor material) is about 120.
46. Inductive Transducer
A passive transducer is said to be an inductive transducer, when it produces the
variation (change) in inductance value. the following formula for inductance, L of an
inductor.
Reluctance
47. Differential transformer
It consists of a transformer with one primary and two secondary windings. The secondary
windings are connected so that their induced voltages oppose each other. If the core is in
the center position, as shown in the figure, the voltages in the two secondary windings are
equal in magnitude and the resulting output voltage is zero. If the core is moved upward as
indicated by the arrow, the voltage in secondary 1 increases while that in secondary 2
decreases. The magnitude of the output voltage changes with the amount of displacement
of the core from its central or neutral position. Its phase with respect to the voltage at the
primary winding depends on the direction of the displacement. Because nonlinearities in
the magnitudes of the voltages induced in the two output coils tend to compensate each
other, the output voltage of the differential transducer is proportional to core
movement even with fairly large displacements.
Mathematically, the output voltage, 𝑉0 can be written as
V0=VS1−VS2
48. Passive Transducers Using Capacitive Elements
The capacitance of a plate capacitor can be changed by varying the physical dimensions
of the plate structure or by varying the dielectric constant of the medium between the
capacitor plates. Both effects have occasionally been used in the design of transducers
for biomedical applications. The capacitance plethysmograph shown in Chapter 6 is an
example. As with the transducers using an inductive element, it is sometimes not
apparent whether a capacitive transducer is of the passive type or is actually an active
transducer utilizing the principle of electric induction. If there is doubt, an examination
of the carrier signal can help in the classification. Passive transducers utilize ac carriers,
whereas a dc bias voltage is used in transducers based on the principle of electric
induction.
